This invention relates to lasers, and more particularly to an improved gas laser which includes a pair of coupled laser cavities with a gaseous gain medium in one cavity and an active modulator in the coupled cavity, such that continuous wave (cw) energy as well as various forms of pulsed laser energy at different levels may be selectively coupled out of the coupled cavity through the action of the modulator.
Carbon dioxide lasers have been used in the past to produce IR energy in the 10 micron wavelengths region. The output of such lasers can be actively modulated by modulators which are either within the laser cavity or external thereto. Such techniques include acousto-optic modulation for extracavity frequency shifting and intracavity mode locking, electro-optic modulation for mode locking, Q-switching and cavity dumping, chirp modulation, Stark effect modulation for Q-switching and cavity dumping, and intracavity electro-optic modulation. An example of the latter technique is found in U.S. Pat. No. 4,176,327.
Intracavity electro-optic modulation offers the widest diversity in modulation formats for a modulated CO.sub.2 laser transmitter, considering efficiency, modulation rate, power handling capacity, and modulation range. Also, the use of an intracavity modulator arranged to modulate the laser energy by changing the intracavity polarization or coupling is considered to offer the widest range of modulation formats. IR lasers with multiple modulation formats are extremely useful as laboratory signal sources and in such applications as multi-function laser radars. For example, an airborne laser radar may require widely different transmitted signals to perform such diverse functions as target acquisition, range contouring, range rate measurement, Doppler navigation, three dimensional imaging, etc.
Intracavity modulation is preferred over extracavity modulation because of the lower modulator voltages required to produce a given depth of modulation. Further, recent advances in solid state devices have made available extremely low loss cadmium telluride (CdTe) electro-optic modulators for the 10 micron region. Earlier modulators of this type have exhibited an insertion loss which made intracavity modulation impractical for many applications.